The present invention relates to composite articles such as fiber resin composites having a polymer matrix embedding individual fibers, and composite prepregs having individual fibers with a coating of polymer particles. The invention further relates to methods for making such composite articles, such as the use of an emulsion with polymer particles of sufficiently small size to penetrate gaps between individual fibers. The invention also relates to other composite articles in which a porous material has a polymer matrix embedded within pores of the material.
There has been a long-felt need for lightweight fiber-resin composite materials which display greater strength than currently known composites. Fiber resin composites include fibrous articles, sheets and strands (e.g., tow or yarn) which incorporate a polymer matrix embedding fiber bundles or individual fibers of the article, sheet or strand. One major application of these composites are materials for military airplanes.
There are two major types of fibers used in compositesxe2x80x94chopped glass fibers and continuous fibers. Chopped glass fibers are used to make composites of relatively lower strength. These composites contain from 20% to 40% of fiber by volume, usually as a mat, as described in U.S. Pat. No. 3,713,962. Stiffer and/or stronger composites use continuous fibers in yarn form and contain more than 50% fiber by volume. Examples of stiffer fibers include graphite, polyaramid or special glass fibers. As the volume of fiber in the composite increases, obtaining a uniform matrix between the fibers tends to be more difficult.
Composites are often prepared via a xe2x80x9cprepreg,xe2x80x9d i.e. a composite precursor, in which the fibrous articles or strands are impregnated with a polymer matrix precursor mixture (e.g. U.S. Pat. No. 3,784,433). Prepregs are typically placed in a mold with the fibers positioned in a desired sequence and orientation and subsequently heated under pressure to fuse or polymerize the precursor components to form the polymer matrix of the final composite. The prepreg allows control of the resin content and fiber orientation. Prepregs can be provided as collimated tapes or fabrics.
One type of polymer matrix comprises thermoset plastics or resins. Thermoset resins typically are prepared from precursor mixtures comprising an oligomer and a crosslinking reagent. When heat or energy is applied, the precursor mixture reacts to form a hard, three-dimensional, cross-linked polymer matrix. Incorporating thermoset resins in composites is a relatively facile process because the starting components of thermosets are either liquid resins or solutions of the precursors. These are low viscosity liquids from 100 to 5,000 centipoise (0.1 to 5 Paxe2x80xa2s), which can rapidly wet the fibers. Yarns of glass, graphite or polyaramid are easily penetrated by the low viscosity resin to the core of the yarn, thus providing each fiber with a complete coating of polymer.
Thermoset composites suffer from several disadvantages. Low molding pressures are used to prepare these composites to avoid damage to the fibers. These low pressures, however, make it difficult to suppress the formation of bubbles within the composite which can result in voids or defects in the matrix coating. Thus, most processing problems with thermoset composites are concerned with removing entrained air or volatiles so that a void-free matrix is produced. Thermoset composites made by the prepreg method require lengthy cure times with alternating pressures to control the flow of the resin as it thickens to prevent bubbles in the matrix. Some high volume processes, such as resin infusion avoid the prepreg step but still require special equipment and materials along with constant monitoring of the process (e.g. U.S. Pat. Nos. 4,132,755, and 5,721,034). Thermoset polymers are not easy to process, regardless of whether the resin is applied to the yarns before molding or is infused into a preform of fibers. Although thermoset polymers have enjoyed success as in lower performance composites, the difficulties in processing these resins has restricted their application.
To overcome some of the disadvantages of thermosets, the use of thermoplastic resins as a polymer matrix in composites has been attempted. Thermoplastic resins are long chain polymers of high molecular weight. These polymers are highly viscous when melted and are often non-Newtonian in their flow behavior. Thus, whereas thermosets have viscosities in the range of 100 to 5,000 centipoise (0.1 to 5 Paxe2x80xa2s), thermoplastics have melt viscosities ranging from 5,000 to 20,000,000 centipoise (5 to 20,000 Paxe2x80xa2s), and more typically from 20,000 to 100,000 centipoise (20 to 100 Paxe2x80xa2s). Despite a viscosity difference of three orders of magnitude between thermosets and thermoplastics, some processes have been applied to both types of matrices for laminating fibrous materials.
The combination of high viscosity (thermoplastics) and low pressure (processes to avoid fiber breakage or distortion) is a major source of the molding problems with thermoplastic composites. Due to the high viscosity of thermoplastics, most of the processes to form thermoplastic prepregs involve coating the outside of the fiber bundles with a thermoplastic polymer powder rather than coating individual fibers. The polymer powder is then melted to force the polymer around, into and onto the individual fibers. A few processes apply melt directly to the fibers. A tape can be made by coating a dry tape of collimated fibers with the polymer and applying a heated process that forces the polymer into and around the fibers (e.g., see U.S. Pat. Nos. 4,549,920 and 4,559,262). These processes involve a polymer of an exceptionally low melt viscosity, such as polyetherketone (PEEK), as described in U.S. Pat. Nos. 4,883,552 and 4,792,481.
Other processes for incorporating thermoplastics in composites involve preparing a thermoplastic slurry and melting and forcing the slurry onto the yarn (U.S. Pat. No. 5,019,427). A few thermoplastics can be dissolved and introduced into the fiber bundle as a solution. Removal of solvent presents extra processing problems, however. Alternatively, U.S. Pat. No. 5,725,710 describes pretreating the fibers with a dilute dispersion to ease the passage of the melt polymer in a subsequent pultrusion step to make a tape prepreg. Another process involves commingling, in which structural fibers such as graphite or glass are mixed with a thermoplastic fiber and the subsequent hybrid yarn is woven into a fabric to be molded later (e.g., see U.S. Pat. Nos. 5,355,567, 5,227,236 and 5,464,684). Separate yarns of thermoplastic and reinforcement containing many thousands of filaments, however, cannot be mixed mechanically in a one-by-one arrangement of each fiber. The fibers, at best, are dispersed as smaller bundles. The laminates produced by this process typically contain areas that are resin rich and other areas that are mainly fiber and hence void-containing. Commingled thermoplastics are also restricted to only those polymers which form fibers.
In general thermoplastic composites have had limited success to date, due to a variety of factors including high temperatures, high pressures, and prolonged molding times needed to produce good quality laminates. Most of the efforts have been focused on combining high performance polymers to structural fibers which has only exacerbated the process problems.
One aspect of the present invention is to provide an article, the article comprising a strand of a plurality of fibers and wherein substantially each fiber of the strand is coated by particles of a polymer.
Another aspect of the present invention provides an article comprising a strand of a plurality of fibers, where substantially each fiber of the strand is embedded in a matrix derived from fused polymer particles.
Another aspect of the present invention is a method for forming a composite. The method comprises providing a strand comprising a plurality of fibers and exposing the strand to an emulsion including polymer particles. The method also comprises allowing the particles to form a coating around substantially each individual fiber.
Another aspect of the present invention is to provide a fibrous sheet article comprising a plurality of strands, each strand comprising a plurality of fibers, where substantially each fiber is embedded in a matrix derived from fused polymer particles.
Another aspect of the present invention provides a method for forming a composite fabric. The method comprises providing a fabric comprising a plurality of strands where each strand is a plurality of fibers. Substantially each of the individual fibers of the strands are coated with polymer particles. The method also comprises fusing the polymer particles to form a polymer matrix embedding substantially each fiber.
Another aspect of the present invention provides an apparatus for forming a composite fabric. The apparatus comprises a first roll for supplying a continuous first layer of strands, where each strand of the first layer is aligned along a first direction. The apparatus also comprises at least a second roll for supplying a continuous second layer of strands positioned adjacent the first layer thereby forming a fabric. Each strand of the second layer is aligned along a second direction, where the second direction is different from that of the first direction. The apparatus further comprises a reservoir containing an emulsion including polymer particles where the particles are capable of coating substantially each individual fiber of the strands of the fabric. A conveyor may also be provided to carry the fabric to and from the emulsion reservoir.
Another aspect of the present invention provides a method for forming a composite. The method involves providing an article having pores and exposing the article to a polymer emulsion. The particles of the polymer are allowed to impregnate the pores of the article.
Another aspect of the present invention provides a composite comprising a porous article having polymer particles impregnating the pores of the article.
Another aspect of the present invention provides a composite comprising a porous article having a polymer matrix embedded within the pores of the article.
Other advantages and features of the invention will be apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.